74 GEOLOGICAL SURVEY GF THE TERRITORIES. 
moderately gibbous, the valves having a nearly regular convexity, flat- 
tened or a little concave along the postero-dorsal portion; moderately 
concave in front of the beaks, where there is an almost defined lunule ; 
test thick or even somewhat massive in old shells; dorsal outline forming 
a somewhat regular convex curve from the beaks to the postero-dorsal 
portion, the border of which latter portion is abruptly, sometimes almost 
angularly, rounded to the lower margin; the latter margin forming an 
approximately true semicircular curve from the posterior to the antero- 
cardinal margin, but the convexity of this curve is in old shells a little 
greatest about its middie; antero-cardinal margin straight or slightly 
concave, meeting the antero-basal margin at an obtuse angle or promi- 
nent, abrupt curve; beaks prominent, elevated, curving inward and 
forward, and, when well preserved, ending in a well-defined point; lateral 
teeth strong, well developed, and finely crenulate; cardinal teeth well 
developed, the outer posterior one showing, in one example at least, 
faint crenulations, but otherwise they are of the ordinary character; pal- 
lial line distinct, somewhat distant from the margin; pallial sinus small, 
directed str ongly upward. 
Surface marked only by the ordinary lines and undulations of growth. 
Height of the largest example in the collection, 42 millimeters ; extr eme 
transverse length ‘about the same; thickness, both valves together, 32 
millimeters. The largest examples are all more or less broken; those 
figured on plate 25 being of smaller size. 
This species bears, perhaps, more resemblance to C. cytheriformis Meek 
& Hayden, than to any other published form; but it may be distin- 
guished from the typical forms of that species by its more distinctly 
subtrihedral outline, its greater proportionate height and its concave, 
almost lunulate front. It bears some resemblance also to C. occidentalis, 
especially to some examples of the variety of that species which occars 
in the valleys of Bitter Creek and Yampa Rivers, to be noticed on a fol- 
lowing page. 
Position and locality —lLaramie Group, valley of Crow Creek, North- 
ern Colorado, about 15 miles above the confluence of that creek with 
- Platte River. 
CORBICULA CYTHERIFORMIS Meek & Hayden. 
Plate 21, figs. 4 a, b, c, and d. 
The type specimens of this species were discovered by Dr. Hayden in 
the Laramie strata of the Judith River series. It is described and fig- 
ured by Meek in vol. ix U.S. Geol. Sur. Terr. (4to ser.), p. 520, pl. 40, figs. 
5 a,b, ¢,d,ande. It has been recognized also in the Bitter Creek series, 
the specimens which are illustrated on plate 21 having been obtained 
from a locality about two miles west of Point of Rocks Station, Union 
Pacific Railroad, Wyoming. 
CORBICULA NEBRASCENSIS Meek & Hayden. 
Dr. Hayden obtained this form also from the Laramie strata of the 
Upper Missouri River region; and it has not yet been recognized at 
any other locality. It is “described and figured by Meek in vol. De 10% 
S. Geol. Sur. Terr. (4to ser.), p. 522, plate 43, figs. 2 a and b. 
